gst/base/gstbasesink.c: Refcounting fixes.

gst/base/gstbasesink.c: Refcounting fixes.

Original commit message from CVS:
* gst/base/gstbasesink.c: (gst_base_sink_pad_getcaps),
(gst_base_sink_pad_setcaps), (gst_base_sink_pad_buffer_alloc),
(gst_base_sink_preroll_queue_flush), (gst_base_sink_handle_object),
(gst_base_sink_event), (gst_base_sink_do_sync),
(gst_base_sink_chain), (gst_base_sink_loop),
(gst_base_sink_deactivate), (gst_base_sink_activate_push),
(gst_base_sink_activate_pull), (gst_base_sink_change_state):
Refcounting fixes.
Fix logic for returning ASYNC when not prerolled.